In the past, an optical measurement method using flow cytometry (flow cytometer) has been used for analyzing biologically-relevant microparticles such as cells, microorganisms and liposomes. Flow cytometers are devices to irradiate, with light, particles flowing through a flow channel formed in a flow cell, a microchip or the like, and to detect and analyze fluorescence or scattered light emitted by each of the particles.
Some flow cytometers have a function of separating and collecting only particles having specific properties on the basis of the analysis result. Particularly, such flow cytometers to sort cells are called “cell sorters.” Generally, in a cell sorter, a fluid is broken into droplets upon discharged from a flow channel by causing a vibration element or the like to vibrate a flow cell or a microchip (see PTLs 1 and 2). After positively (+) or negatively (−) charged, each droplet isolated from the fluid is caused to travel in a direction deflected by deflection plates or the like, and collected in a predetermined container or the like.
However, in a droplet sorting device such as a cell sorter, sorting performance is prone to be unstable when affected by temperature change, fluid pressure fluctuation, differential pressure fluctuation occurring after sheath pressure change, and the like. Accordingly, in the past, to stabilize sorting performance, there has been proposed a microparticle sorting device configured to control a driving voltage supplied by a voltage supply unit by imaging a fluid and droplets ejected from an orifice of a flow cell or a microchip, and then by detecting the droplets from the image (see PTL 3).